Geothermal heating and air-conditioning units have the advantage over conventional forced air and boiler units of utilizing the near constant temperature of the earth as a heating/cooling sink. This is usually done by the circulation of a thermally conducting fluid, typically water mixed with propylene glycol, ethylene glycol, methanol or similar anti-freezing compound, through a sealed geothermal loop field buried deep underground. In the northern tier of the United States, it is often necessary for the thermally conducting fluid to include as much as 25% by volume of an anti-freezing compound to prevent the formation of damaging ice crystals in the fluid. After having passed through the ground, the cooled or warmed thermally conducting fluid is then passed through a heat exchanger, which extracts heat from the fluid or passes unwanted heat from the interior of the building to the fluid (depending upon whether the unit is in its heating or cooling mode). After passing through the heat exchanger, the heated or cooled thermal conducting fluid is again pumped back into the ground where it can once again extract heat or dissipate heat (again, depending upon heating or cooling) into the earth. Such geothermal heating and air-conditioning units provide improved energy efficiency relative to traditional forced air heating systems. This efficiency advantage can, in some cases, generate a 30% decrease in energy expenditures compared to traditional forced air heating and air-conditioning systems.
In order to install geothermal heating and air-conditioning systems, a geothermal loop field comprised of a series of connected, thermally conductive pipes is typically installed in the earth to contain the thermally conducting fluid. The prior art vertical geothermal loop fields were typically installed into a very deep, spaced apart boreholes drilled into the ground near the building to be heated and/or cooled. For a three thousand square foot residence, prior art systems typically required between about 2 and about 4 boreholes, (depending on the region) spaced at least 15 feet apart. As a general rule of thumb, prior art geothermal heating/cooling systems typically included at least 300 lineal feet of geothermal vertical borehole depth (vertical loop depth) for every 1000 square feet of living space in the residence. Commercial geothermal systems are typically much larger and required a significantly larger number of boreholes. One inefficiency created by the prior art loop fields was that the set up process for the drill rig for each new borehole was more labor intensive and time consuming than is desirable. Such processes typically involve a crew of two to three workers and usually required about ninety minutes of their time. Further prior art loop field drill rigs were often not usable when the temperature dropped below freezing since the drilling recirculation fluid often freezes at such temperatures.
In order to minimize the number of time consuming drill rig breakdown and set up processes for a given prior art loop field, typically each field was designed with the minimum number of deep boreholes that would provide the required number of lineal feet of vertical loop necessary for the building. For this reason, most residential loop fields utilize boreholes that are drilled between about 300 to about 600 vertical feet into the ground. In order to create the deep boreholes for the prior art loop field, large drill rigs weighing from about 7 to 25 tons were typically used and were either towed between the spaced apart borehole positions by large trucks or the drill rig itself was integrated into a heavy duty drill rig truck. Each of the boreholes of a typical prior art geothermal loop field descend through an average of about 100 feet of overburden (soil and other relatively loose material above the bedrock horizon) and usually through at least about 200 feet of bedrock. The depth of overburden above bedrock at a particular location can vary considerably and varies significantly between different portions of the same county, state, etc. Since boring the typical residential four-inch borehole through bedrock requires at least about 4000 pounds of down pressure at the drill bit, the large heavy drill rigs of the prior art were deemed necessary to provide such down pressures and to provide sufficient power to lift and drive into the hole the pipe necessary to drill boreholes. The deep, prior art vertical boreholes are typically joined together by number of horizontal pipe runs that are routed to a manifold located on the exterior of the structure to be heated and/or cooled. From this exterior, buried manifold, the thermally conducting fluid is piped to interior of the structure to the condenser unit. Installation of the horizontal pipe runs and manifold requires a significant amount of trenching and backfilling to bury the horizontal pipe runs and manifold underground. Another problem is that such horizontal pipe runs needed to be buried in deep trenches that were below the local frost line. Further, installation of prior art vertical loop fields typically required drilling through the exterior wall (typically made of concrete) of the building foundation to allow the pipes entering and exiting the manifold to access the building interior. These trenching, backfilling, and building foundation drilling processes add significant cost to the construction of a prior art geothermal loop field. The buried manifolds of the prior art systems are difficult to purge, and if not purged correctly, negatively affect the efficiency of the system. Further, periodic maintenance and servicing of the geothermal system will sometimes require access to the buried manifold so it may have to be dug up on occasion, which can be inconvenient for the property owner.
The applicant has discovered that the deeply bored prior art geothermal loop fields are less efficient in operation and more costly to construct than is desirable. For example, during the latter portions of the winter in the northern tier of the continental United States, the applicant has found that prior art geothermal loop fields frequently return thermal fluid to the condenser that was only a few degrees above freezing or in some case several degrees below 32° F. During periods of such near freezing thermal fluid return, the efficiency of the geothermal unit for heating was significantly degraded. As a general rule of thumb, the warmer the return fluid during the heating season, the more BTU's per hour the system can produce. In part, this is because at such low fluid return temperatures, the thermal conducting fluid becomes sufficiently viscous that it causes the system's pump to use excessive amounts of energy to keep the fluid circulating. Furthermore, the large drill rig equipment required for drilling prior art geothermal field loops was often too large to maneuver into place for drilling in small, closely spaced lots, particularly, urban residential lots. Urban residential lots often have garages, utility poles, trees and encroaching buildings on adjacent lots that leave only a very small pathway to access any open space within the lot that is appropriate for drilling boreholes. For this reason, such lots were often too congested to allow prior art drill rigs to reach the interior of the lot for drilling a sufficient number of boreholes to provide enough heating/cooling capacity to meet the needs of the building owner. Due to this situation, owners of smaller lots, including many small lot urban homeowners, have been dissuaded from even considering geothermal heating and air-conditioning solutions by their builders or remodeling contractors.